Electrospray interfaces are commonly used to receive liquid from a liquid sample source such as a liquid chromatograph ("LC") and to produce ions which are delivered to a mass spectrometer. In electrospray, liquid from the LC is directed through a free end of a capillary tube, the tube being connected to one pole of a high voltage source. The free end of the capillary tube is spaced from an orifice plate having an orifice leading (directly or through other equipment) into the mass analyzer vacuum chamber. The orifice plate is connected to the other pole of the high voltage source. The electric field generates a spray of charged droplets, producing liquid flow without a pump, and the droplets evaporate to produce ions.
Because electrospray can handle only a very small flow (larger flows produce larger droplets, causing the ion signal to fall off and become unstable), a new technique was developed, which can be referred to as nebulizer gas spray. The nebulizer gas spray technique, shown in U.S. Pat. No. 4,861,988 issued Aug. 29, 1989 to Cornell Research Foundation, involves providing a cocurrent flow of high velocity nebulizer gas coaxial with the capillary tube. The nebulizer gas nebulizes the liquid to produce a mist of droplets which are charged by the applied electrical field. While electrospray functions poorly at liquid flows over about 10 microliters per minute, nebulizer gas spray functions reasonably well at liquid flows of up to between 100 and 200 microliters per minute. However even with nebulizer gas spray, it is found that with liquid flows of the order of about 100 microliters per minute, the sensitivity of the instrument is less than at lower flows, and that the sensitivity reduces substantially at liquid flows above about 100 microliters per minute. It is believed that at least part of the problem is that at higher liquid flows, larger droplets are produced and do not evaporate to release ions before these droplets reach the orifice. Therefore, much sample is lost.
Various attempts have been made to improve the sensitivity of instruments using nebulizer gas spray and electrospray. For example, as shown in U.S. Pat. No. 4,935,624 issued Jun. 19, 1990, attempts have been made to heat the liquid before it is sprayed through the capillary tube. However because heating the liquid in the capillary tube to a high temperature will degrade thermally labile analytes, this method is not desirable and has produced only a limited increase in sensitivity.
In another attempt to improve the results when using electrospray, two researchers at the University of Alberta in Alberta, Canada, namely Paul Kebarle and Michael Ikonomou, have recently suggested surrounding an electrospray capillary with a cocurrent coaxial sheath of heated entraining gas. The flow used is not a nebulizing flow, but rather is a laminar flow of heating gas. It is found that this can increase the sensitivity of the instrument by 3 to 5 times, but in practice the device has proven temperamental and the improvement is available only within a very narrow range of operating parameters.